This invention relates to a waste heat recovery boiler which uses an exhaust gas of a gas turbine as a heat source and generates a driving steam for other steam power prime movers, and more particularly to a waste heat recovery boiler capable of reducing the concentration of nitrogen oxides NOx in the exhaust gas.
In a conventional composite cycle plant equipped with a gas turbine, a denitrating device is disposed in a waste heat recovery boiler which generates steam with the exhaust gas of the gas turbine as its heat source and a steam turbine using the generated steam as its driving steam, with the denitrating device removing NOx from the exhaust gas.
Generally, the denitration ratio of the denitrating device increases with a higher reaction temperature within a predetermined temperature range. For example, in a dry catalytic reduction decomposition process which charges ammonia as a reducing agent into the exhaust gas and passes the gas through a reactor to which an iron oxide type catalyst is packed in order to reduce and decompose the nitrogen oxides into non-toxic nitrogen contents and a steam, the denitration ratio depends greatly upon the reaction temperature of the catalyst layer, that is, the combustion gas temperature which is passed through the catalyst layer of the denitrating device. The denitration ratio drops drastically when the reaction temperature drops from 300.degree. C. to 200.degree. C., and substantially the highest denitration efficiency is attained at temperatures ranging from 310.degree. C. to 470.degree. C.
The temperature of the exhaust gas from a gas turbine varies widely between operating at rated load and operating under partial load. For example, the temperature drops from about 530.degree. C. when operating at rated load to about 330.degree. C. when operating with a 25% load. Therefore, Japanese Patent Laid-Open No. 96604/1979 proposes dividing a high-pressure evaporator, which is disposed inside the waste heat recovery boiler so that the temperature of the exhaust gas is as rear as possible to the predetermined temperature range described above even when the load condition of the turbine changes, into a high-pressure evaporator and a low-pressure evaporator.
Even if such a method is employed, however, the exhaust gas temperature at the inlet of the denitrating device drops down to about 260.degree. C. at 25% of rated load; hence, a reduction of the efficiency of the denitrating device is unavoidable. In order to hold the NOx concentration at the outlet of the denitrating device below a certain value, the effect of the catalyst of the denitrating device must be drastically increased, although such a need does not exist during operation at rated load. Furthermore, although the NOx concentration in the exhaust gas of the gas turbine changes with the load, the conventional apparatus described above can not adequately compensate for such a change.
It is an object of the present invention to provide a waste heat recovery boiler which can reduce NOx with a minimal quantity of catalyst, and can arbitrarily set the distribution of denitration efficiency in accordance with a load to maintain NOx at prescribed values even when the load of a gas turbine changes from a low load to the rated load.
In a waste heat recovery boiler which generates steam for driving a steam turbine by utilizing the exhaust gas of a gas turbine, the present invention is characterized in that at least two denitrating devices for removing NOx components in the exhaust gas are disposed in the waste heat recovery boiler so as to change the distribution of the denitration efficiency of the denitrating device with respect to the load, and can thus avoid any adverse effects resulting from the change of exhaust gas temperatures with the change of the NOx concentration distribution in the exhaust gas of the gas turbine and with the change of the exhaust gas temperature.